Robert Letham’s Union with Christ is a good overview of a subject that is vital for understanding how we receive all the benefits Christ procured through earthly life and death. One of the things I appreciate about his approach is the way he defines “union in Christ” in covenantal categories. Even if appeals to the classical “covenant of grace” / “covenant of works” approach to the biblical covenants, his approach rightly assigns “union in Christ” to a covenantal concept.
Of late, I have heard some people speak about “union in Christ” and Christ’s mediation (a la 1 Timothy 2:5) without paying attention to the biblical idea of the covenants. Letham corrects this sort of approach. He shows how “union in Christ” cannot be explained our understood apart from understanding Christ as a “covenant head” and someone who is united to us in a “covenantal” relationship. Here is how he summarizes his understanding of Union in Christ:
(1) Union with Christ is based on Christ’s being our covenant head and is established by his sharing our nature.
(2) Since he is our head and representative, who shares our humanity, all that he did in his earthly ministry was done as a substitute and representative.
(3) Yet our union with Christ goes much further than this. Since he shares our nature, and since the Holy Spirit unites us to him, all that he did and does is in union with us. He took our place under the wrath of God, while we take his place as sons of the Father. He is the captain of the team of which we are members. When the captain scores the winning goal in the final minute of stoppage time, the whole team participates in the captain’s actions.
(4) This union is the ground of our whole salvation, justification included. We receive a right status before God, since we are incorporated into the Son of God himself. All that he did is ours (Robert Letham, Union with Christ, 83).
And earlier in his book, Letham writes,
The central promise of God’s covenants is the repeated statement in each covenantal administration, “I will be your God; you shall be my people.” It is seen in Genesis 17:7-8 in the Abrahamic covenant, in Jeremiah 11:4 in connection with the Mosaic covenant, in Jeremiah 24:7 with the return from the Babylonian exile in view, and in Jeremiah 30:22; 31:33; and 32:38 pointing forward to the new covenant. It features also in Revelation 21:3 in the vision of the glorious bride of Christ. This is the heart of all of God’s covenants, the promise of God and his people at one in covenant fellowship (Robert Letham, Union with Christ, 36).
Thus, all of God’s historical covenants are centered in Christ and fulfilled by him. Moreover, they are centered in union with Christ—it is in union with him that he is our God and we are his people (Robert Letham, Union with Christ, 37).
For more on this subject, see my quotations page: Union in Christ.
Soli Deo Gloria, dss